1. Field of the Invention
The present invention relates in general to acoustic information generation machines and more particularly to damping devices and acoustic vibration filters for use with acoustic information generation machines.
2. Background of the Invention
In the past, acoustic vibration has been utilized to carry information in a variety of circumstances. A classic example of an acoustic information generation machine is a phonograph record player. A phonograph record has grooves with predetermined waves formed therein. These waves carry information, such as music, which is transmitted to a stylus in the form of acoustic vibration of the stylus. Rotation of the phonograph record so that the stylus tracks in the grooves generates the acoustic information which is then ultimately converted to an amplified sound.
Another example of an acoustic information generation machine is a testing device which generates cracks in a pipe so that the crack initiation propagation can be recorded. The information on crack behavior in the pipe material is useful in evaluating the pipe and the pipe material. Machines of this type apply a stress on a pipe to initiate and propagate a crack in the pipe. Typically, a hydraulic piston is hydraulically actuated to exert a radial force on a midportion of the pipe. The ends of the pipe are supported to oppose this force. As the crack propagates due to this bending force, the pipe emits accoustic signals and this information is recorded and analyzed to evaluate the crack propagation.
A particular problem in acoustic information generation machines of the past has been isolating extraneous acoustic signals from the machine. For example, in the playing of a phonograph record, it is important that the only vibrations transmitted through the stylus from the record be those produced by the waves in the grooves. If the phonograph record is vibrated from outside sources, these vibrations will be transmitted to the stylus distorting the music which is amplified. A particularly troublesome source for these extraneous vibrations is the motor which rotates the phonograph record and the bearings which support the phonograph record and turntable during rotation.
In the testing device described above extraneous vibration is often recorded along with the signals produced by the crack propagation distorting the information concerning crack behavior. A typical source for the extraneous vibration in the testing device is the hydraulic piston which stresses the pipe. Other sources of extraneous acoustic emission are the bearing surfaces of the pipe supports.
In the past, a typical solution for reducing the amount of extraneous acoustic vibration transmitted to the acoustic information generation machine is to introduce a rubber piece between the extraneous source of vibration and the acoustic information generation point. For example, in phonograph record players often the motor is connected to the phonograph record turntable through a rubber belt which extends from a motor shaft to a turntable shaft. In this way, the motor vibration is isolated from the turntable. However, a disadvantage of using rubber is that rubber tends to wear out quickly, requiring replacement of the rubber belt. Furthermore, the rubber can become stiff with age, reducing its ability to isolate the extraneous vibration. When too resilient, the rubber belt can stretch or slip causing the turntable platter to lag behind the motor. Finally, a rubber belt positioned as described above does not isolate the bearing vibration transmitted from the turntable shaft bearing to the turntable.
In the test device rubber is an unsatisfactory way to isolate the extraneous acoustic vibration from the test piece because rubber cannot withstand the stresses necessary to produce a crack propagation in the pipe. Accordingly, an alternative method has been used to prevent the recording of erroneous information. This alternative consists of positioning acoustic sensors (for example, transducers) adjacent each of the supports and the hydraulic cylinders so that acoustic signals coming from these sources will be detected. By simultaneously comparing the signals received by sensors located away from these points, it can be determined whether the signal is being received solely from the crack propagation or from extraneous sources as well. When extraneous sources are producing vibration which distorts the recording of crack propagation vibration, the recording can be stopped. Thus, only true crack propagation acoustic vibration is recorded.
A disadvantage with the "lock-out" transducers or sensors used for halting the recording during extraneous vibration is that only part of the crack propagation information is received. That information generated during a non-recorded period is lost. Furthermore, this method of using lockout transducers requires expensive equipment to compare the acoustic information from the lock-out transducers with the acoustic information received by the crack propagation transducers.
Recently, shape-memory alloys have been discovered which have the unusual property of recovering their predeformation shape after they have been heated from a lower temperature at which a deformation occurred. This material is described in U.S. Pat. No. 4,149,911 to Clabburn. A particular, although unexplained, property of many of these shape-memory alloys is that they damp vibrations. These damping characteristics are particularly high in the austenitic phase of the alloy. In the past, however, shape-memory alloys have not been utilized for a damping material.